The present invention relates generally to saving power in digital subscriber line (DSL) modems, and specifically to a level two method and system for providing power savings.
Remote access and retrieval of data is becoming increasingly popular in data communication. The proliferation of the Internet has provided a vast network of information that is available to the general public. As the Internet grows and technology advances, this information is becoming increasingly voluminous and the details are becoming increasingly intricate. The increase in volume of information to be transferred has presented a need for a high-speed Internet connection, since traditional telephone modems communicate at speeds too slow for efficient communication.
One proposal for high-speed communication is the introduction of digital subscriber line (DSL) technology. One of the most attractive features of DSL is that it is implemented using an infrastructure that already exists. DSL shares copper twisted pair lines typically used for telephone communication. However, only a small portion (0 to 4 kHz) of bandwidth on the twisted pair line is used for Plain Old Telephone Service (POTS). DSL takes advantage of additional available bandwidth (from 4 kHz to approximately 1.1 MHz) for transmitting data. There are various forms of DSL, generally referred to as xDSL, including Asymmetric DSL (ADSL), High bit-rate DSL (HDSL), Symmetric DSL (SDSL), Symmetric High bit-rate DSL (SHDSL), Integrated Services Digital Network (ISDN) DSL systems and the like.
ADSL is currently the most practical form of DSL technology, and therefore the most widely implemented. ADSL is asymmetric in that its downstream (to a subscriber) capacity is larger than its upstream (from the subscriber) capacity. An ADSL transceiver unit at a central office or remote loop carrier (ATU-C) is used for sending downstream information and receiving upstream information. An ADSL transceiver unit at a remote location or user end (ATU-R) is used for receiving downstream information and sending upstream information. Typically, a Discrete Multi-tone (DMT) scheme is used. The spectrum from 4 kHz to 1.1 MHz is divided into 255 sub-channels, or tones, each having a bandwidth of 4.3125 kHz. Each sub-channel uses Quadrature Amplitude Modulation (QAM) to carry 1 to 15 bits per QAM symbol. When the modems initialize at power-up they attempt to train and synchronize their signals with the other side.
It is of great interest for DSL service providers to achieve power saving on DSL line cards on the central office (CO) side. The benefit of power saving translates into higher line density, less stringent power supply and heating requirements, and a smaller installation footprint, which are especially important for remote DSL deployment.
There are presently two underlying approaches for quiescent-mode (Q-mode), which is a transparent power savings mode. The first approach is referred to as a suspended operation approach. The second approach is referred to as a free-running operation approach. Each of these approaches and their shortcomings are outlined in the following paragraphs.
For the suspended operation approach, all of the clocks, registers, interleavers, states, and the like in higher layers are frozen upon entry into Q-mode, and are restarted upon exit. During operation in Q-mode, a special low power Q-mode signal is transmitted in place of the user data modulated signal. There are a number of problems associated with the above proposals. There is an uncertainty to the amount of power reduction actually obtained. A drift in frequency domain equalizer is possible due to a difference in the Q-mode signal as compared to a Showtime signal. A correlator and a synchronized pseudo-random binary sequence (PRBS) generator are needed at a receiver to detect an exit signal. The transmitter needs to communicate the Q-mode line signal to the receiver during initialization. Lastly, there are questions about the robustness of the approach's exit mechanism.
For the free-running approach, the transmission communication (TC) layers are modified to generate an all zero sequence when there is no data to transmit. The physical (PMD) layer transmits a low power QPSK signal in response to the unscrambled, all zero sequence. Exit is via a special symbol, and resynchronization between the transmitter and receiver is allowed if an exit message is missed. However, a drawback to the free-running approach is the need to modify all of the transmission communication (TC) layers to generate an all zero sequence when there is no data to transmit. This results in additional coupling between the physical and TC layers, which is preferable to avoid. In addition, the position of the data scrambler has to be moved to allow the all zero data signal at the PMD layer.
Thus, it is an object of the present invention to provide a method and system for providing a power savings that obviates or mitigates at least some of the above mentioned disadvantages.